This invention relates to optical pressure sensing and, more particularly, to optical pressure sensing based on Bragg gratings imparted in an optical fiber.
In the extraction of oil from earth borehole, the naturally existing pressure within an earth formation is often used as the driving force for oil extraction. The oil may be extracted from a single location or from multiple locations within the well. In either case, it is desirable to know the fluid pressure with the well at one or more locations to aid the well operator in maximizing the depletion of the oil within the earth formation.
It is known to install pressure and temperature sensors with some electrical submersible pumps (ESPs) to provide the operator on the surface with information about the pump""s performance. It is also known to use optical sensors for the measurement of wellbore conditions such as downhole wellbore pressures and temperatures. FIG. 1 illustrates such an environment. As shown in FIG. 1, the pressure sensor can be mounted to the casing of an electrically submersible pump. A light source in an optical module is used to feed optical signals to the pressure sensor through the optical fiber assembly. The signal indicative of the pressure at the sensing location provided by the pressure sensor is conveyed back to the optical module for processing. For pressure sensing at multiple locations within the wellbore, multiple pressure sensors may be serially multiplexed for distributed pressure sensing using wavelength division multiplexing (WDM) and/or time division multiplexing (TDM) techniques.
Sensors for the measurement of various physical parameters such as pressure and temperature often rely on the transmission of strain from an elastic structure (e.g., a diaphragm, bellows, etc.) to a sensing element. In a pressure sensor, the sensing element may be bonded to the elastic structure with a suitable adhesive.
It is also known that the attachment of the sensing element to the elastic structure can be a large source of error if the attachment is not highly stable. In the case of sensors, which measure static or very slowly changing parameters, the long-term stability of the attachment to the structure is extremely important. A major source of such long term sensor instability is a phenomenon known as xe2x80x9ccreepxe2x80x9d, i.e., change in strain on the sensing element with no change in applied load on the elastic structure, which results in a DC shift or drift error in the sensor signal.
Certain types of fiber optic sensors for measuring static and/or quasi-static parameters require a highly stable, very low creep attachment of the optical fiber to the elastic structure. One example of a fiber optic based sensor is that described in U.S. Pat. No. 6,016,702 entitled xe2x80x9cHigh Sensitivity Fiber Optic Pressure Sensor for Use in Harsh Environmentsxe2x80x9d, issued to Robert J. Maron, which is incorporated herein by reference in its entirety. In that case, an optical fiber is attached in tension to a compressible bellows at one location along the fiber and to a rigid structure (or housing) at a second location along the fiber with a Bragg grating embedded within the fiber between these two fiber attachment locations. As the bellows is compressed due to an external pressure change, the strain on the fiber grating changes, which changes the wavelength of light reflected by the grating. If the attachment of the fiber to the structure is not stable, the fiber may move (or creep) relative to the structure it is attached to, and the aforementioned measurement inaccuracies occur.
One common technique for attaching the optical fiber to a structure is epoxy adhesives. It is common to restrict the use of epoxy adhesives to temperatures below the glass transition temperature of the epoxy. Above the glass transition temperature, the epoxy transitions to a soft state in which creep becomes significant and, thus, the epoxy becomes unusable for attachment of a sensing element in a precision transducer. Also, even below the glass transition temperature significant creep may occur.
Another technique is to solder the structure to a metal-coated fiber. However, it is known that solders are susceptible to creep under certain conditions. In particular, some soft solders, such as common lead-tin (PbSn) solder, have a relatively low melting point temperature and are thus relatively unsuitable for use in transducers that are used at elevated temperatures and/or at high levels of stress in the solder attachment. The use of xe2x80x9chardxe2x80x9d solders with higher melting temperatures, such as gold-germanium (AuGe) and gold-silicon (AuSi), can reduce the problem; however, at elevated temperatures and/or high stress at the solder attachment, these hard solders also exhibit creep. In addition, the high melting temperature of such solders may damage the metal coating and/or damage the bond between the metal coating and glass fiber.
It is advantageous and desirable to provide a reliable method and system for accurately measuring the pressure at one or more locations in an environment, wherein the pressure sensor is comprised of a mechanism to prevent long term sensor instability due to changes in strain on the sensing elements.
The first aspect of the present invention is a pressure sensor, responsive to an optical signal, for providing a sensor signal indicative of pressure in an environment. The pressure sensor comprises an optical waveguide having a longitudinal axis, a first mounting location and a second mounting location separated by a separation distance along the longitudinal axis which propagates the optical sensor signal, wherein the waveguide comprises a core and a cladding disposed outside the core, and wherein the cladding has an outside diameter and includes a first and a second variation region each having a modified outside diameter different from the outside diameter, wherein the first and second variation regions are respectively located at the first mounting location and the second mounting location, wherein a Bragg grating is imparted in the waveguide between the first mounting location and the second mounting location which provides the optical sensor signal having a spectral profile centered at a characteristic wavelength along said waveguide, wherein a first attachment mechanism is disposed against at least one portion of the first variation region which prevents relative movement between the first variation region and the first attachment mechanism, wherein a second attachment mechanism is disposed against at least one portion of the second variation region which prevents relative movement between the second variation region and the second attachment mechanism, and wherein a mounting device has a first end which mounts to the first attachment mechanism and a second end which mounts to the second attachment mechanism which defines a separation length between the first and second attachment mechanisms along the longitudinal axis of the waveguide and allowing the separation length to vary according to the pressure of the environment, thereby causing a change in the separation and the characteristic wavelength.
According to the present invention, the first attachment mechanism comprises a first ferrule including a front portion having a profile substantially corresponding to the modified outside diameter of the first variation region of the cladding, and a first butting mechanism butting the first ferrule against the waveguide which presses the front portion of the first ferrule onto at least one portion of the first variation region at the first mounting location and which limits relative movement between the first ferrule and the first variation region of the cladding, wherein the second attachment mechanism comprises a second ferrule including a front portion having a profile substantially corresponding to the modified outside diameter of the second variation region of the cladding, and wherein a second butting mechanism butts the second ferrule against the waveguide to press the front portion of the second ferrule onto at least one portion of the second variation region at the second mounting location which limits relative movement between the second ferrule and the second variation region of the cladding.
According to the present invention, the mounting device comprises a bellows structure which mounts to the first and second attachment mechanisms at the first and second ends of the mounting device.
In order to isolate the temperature effect on the pressure sensor, a temperature compensation means is used to provide a temperature compensation optical signal.
According to the present invention, the first and second variation regions include expanded regions in the cladding such that the modified outside diameter is greater than the outside diameter of the cladding and the first and second variation regions include recessed regions in the cladding such that the modified outside diameter is smaller than the outside diameter of the cladding.
According to the present invention, a buffer layer may be provided over the cladding which protects the waveguide against the first and second attachment mechanisms and which enhances the attachment of the first and second attachment mechanisms to the waveguide.
According to the present invention, the first and second ferrules comprise a plurality of pieces substantially surrounding the respective variation regions which attach to the cladding.
According to the present invention, it is possible to splice a waveguide segment having a cladding of a different diameter to the waveguide in order to form a variation region. Alternatively, a tube can be fused to the cladding to change the outside diameter of the cladding.
According to the present invention, the waveguide is either held in tension or compression by the attachment mechanisms.
According to the present invention, the waveguide can be heated and stretched, or etched to modify the outside diameter of the cladding.
According to the present invention, the optical waveguide is an optical fiber.
The second aspect of the present invention is a method of sensing pressure in an environment with an optical signal. The method comprises the steps of providing an optical waveguide having a longitudinal axis which propagates the optical signal and provides a sensor signal indicative of the pressure in the environment in response to the optical signal, wherein the waveguide has a first mounting location and a second mounting location separated by a separation distance along the longitudinal axis, and wherein the waveguide comprises a core and a cladding disposed outside the core, wherein the cladding has an outside diameter and includes a first and a second variation region each having a modified outside diameter different from the outside diameter, and wherein the first and second variation regions are respectively located at the first mounting location and the second mounting location, providing a grating imparted in the core of the waveguide between the first mounting location and the second mounting location which provides the optical sensor signal having a spectral profile centered at a characteristic wavelength along said waveguide, providing a first attachment mechanism disposed against at least one portion of the first variation region, which prevents relative movement between the first variation region and the first attachment mechanism, providing a second attachment mechanism disposed against at least one portion of the second variation region, which prevents relative movement between the second variation region and the second attached mechanism, and providing a mounting device having a first end, which mounts to the first attachment mechanism and a second end, which mounts to the second attachment mechanism, which defines a separation length between the first and second attachment mechanisms along the longitudinal axis of the waveguide and allowing the separation length to vary according to the pressure of the environment, thereby causing a change in the separation distance between the first and second variation regions and the spacing of the grating.
According to the present invention, the method further comprises the step of providing a temperature compensation means responsive to temperature of the environment which provides a temperature compensation optical signal.
According to the present invention, the method further comprises the step of providing a coating between the cladding and the first and second ferrules which helps the ferrules to conform with the outside diameter of the respective variation regions and reduces point contact stresses on the waveguide.
According to the present invention, the method further comprises the step of providing buffer layer over the cladding to protect the waveguide against the first and second attachment mechanisms and for enhancing attachment of the first and second attachment mechanisms to the waveguide.
According to the present invention, the method further comprises the step of bonding the buffer layer to the first and second attachment mechanisms.
According to the present invention, the method further comprises the step of splicing a further waveguide segment including a cladding having a second outside diameter substantially equal to the modified outside diameter with the waveguide to form each of the first and second variation regions.
According to the present invention, the method further comprises the step of heating and stretching the waveguide to form the modified outside diameter of the first and second variation regions.
The present invention provides a significant improvement over the prior art by combining an optical fiber having an expanded and/or recessed outer dimension variation region, with a structure, such as a ferrule or housing, having a size and shape such that the structure mechanically locks against at least a portion of the variation region, thereby allowing the structure to attach to the fiber with minimal relative movement (or creep) in at least one predetermined direction between the fiber and the structure. The variation region and the structure may have various different shapes and sizes. However, while the geometry of the variation region is created from the optical fiber, low optical loss of the light being transmitted through the core of the fiber is maintained. There may also be a buffer layer between the cladding and the ferrule to protect the fiber and/or to help secure the structure to the fiber to minimize creep. Adhesives, such as solders, brazes, epoxies, etc., may also be used between the structure and the variation region.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.